The phase transformation in calcite I-IV-V and calcite , aragonite have been characterized by electrical impedance measurements at temperatures 600-1200 C and pressures 0.5-2.5 GPa in a piston cylinder apparatus. The bulk conductivity has been measured from Argand plots in the frequency range 10 5 -10 À2 Hz in an electric cell representing a coaxial cylindrical capacitor. The synthetic polycrystalline powder of CaCO 3 and natural crystals of calcite were used as starting materials. The transformation temperature T c was identified from resistivity-temperature curves as a kink point of the activation energy. At pressure above 2 GPa in ordered phase calcite I, the activation energy E is c. 1.05 eV, and in disordered phase calcite V E is c. 0.75 eV. The pressure dependence of T c for the rotational order-disorder transformation in calcite is positive for pressures <1 GPa and negative for pressures >1 GPa. The transformation boundary of calcite 1-IV is observed only during first heating in samples after a long annealing at low temperatures. The activation energy of calcite I , IV decreases gradually from 1.8 to 1.05 eV with the pressure increase from 0.5 to 2 GPa. The kinetics of calcite , aragonite transformation has been monitored by measuring a time-variation of the electrical resistance of a calcite sample at 10 3 Hz in the stability P-T field of aragonite. The variation of the impedance correlates with the degree of phase transformation, estimated from X-ray powder diffraction studies on quenched products of experiments. The kinetics of calcite ) aragonite transformation may be fitted to the Avrami kinetics with the exponent m $ 1-1.5.